STAR CLOUDS AND NEBULAE
From star clusters to nebulae, only a century ago, the transition was
thought to be easy and immediate. Accuracy in determining the distances
of stars was just beginning to be reached, the clusters were obviously
of all degrees of closeness following to the verge of irresolvability,
and it was but natural to jump to the conclusion that the mystery of the
nebulae consisted in nothing but their vaster distance than that of
clusters, and it was believed that all nebulae would prove resolvable
into stars whenever telescopes of sufficiently great power could be
constructed.
But the development of the spectroscope soon showed the error of this
hypothesis, by revealing bright lines in the nebular spectra showing
that many nebulae emit light that comes from glowing incandescent gas,
not from an infinitude of small stars.
In pre-telescope days nothing was known about the nebulae. The great
nebula in Andromeda, and possibly the great nebula in Orion, are alone
visible to the naked eye, but as thus seen they are the merest wisps of
light, the same as the larger clusters are. Galileo, Huygens and other
early users of the telescope made observations of nebulae, but long-focus
telescopes were not well adapted to this work. Simon Mayer has left us
the first drawing of a nebula, the Orion nebula as he saw it in 1612.
The vast light-gathering power of the reflectors built by Sir William
Herschel first afforded glimpses of the structure of the nebulae, and if
his drawings are critically compared with modern ones, no case of motion
with reference to the stars or of change in the filaments of the nebulae
themselves has been satisfactorily made out.
Only very recently has the distance of a nebula been determined, and the
few that have been measured seem to indicate that the nebulae are at
distances comparable with the stars. Of all celestial objects the nebulae
fill the greatest angles, so that we are forced to conclude, with regard
to the actual size of the greater nebulae as they exist in space, that
they far surpass all other objects in bulk.
Photography invaded the realm of the nebulae in 1880, when Dr. Henry
Draper secured the first photograph of the nebula of Orion.
Theoretically photography ought to help greatly in the study of the
nebulae, and enable us in the lapse of centuries to ascertain the exact
nature of the changes which must be going on. The differences of
photographic processes, of plates, of exposure and development produce
in the finished photograph vastly greater differences than any actual
changes that might be going on, so that we must rely rather on optical
drawings made with the telescope, or on drawings made by expert artists
from photographs with many lengths of exposure on the same object.
The great work on nebulae and star clusters recently concluded by
Bigourdan of the Paris Observatory and published in five volumes
received the award of the gold medal of the Royal Astronomical Society.
While D'Arrest measured about 2,000 nebulae, and Sir John Herschel about
double that number in both hemispheres, Bigourdan has measured about
7,000. His work forms an invaluable lexicon of information concerning
the nebulae.
Classification of the nebulae is not very satisfactory, if made by their
shapes alone. There are perhaps fifteen thousand nebulae in all that have
been catalogued, described, and photographed. Dreyer's new general
catalogue (N.G.C.) is the best and most useful. Many of the nebulae,
especially the large ones, can only be classified as irregular nebulae.
The Orion nebula is the principal one of this class, revealing an
enormous amount of complicated detail, with exceptional brilliancy of
many regions and filaments. An extraordinary multiple star, Theta
Orionis, occupies a very prominent position in the nebula, and
photographs by Pickering have brought to light curved filaments, very
faint and optically invisible, in the outlying regions which give the
Orion nebula in part a spiral character. But the delicate optical wisps
of this nebula are well seen, even in very small telescopes. Its
spectrum yields hydrogen, helium and nitrogen. The Orion nebula is
receding from the earth about eleven miles in every second. Keeler and
Campbell have shown that nearly every line of the nebular spectrum is a
counterpart of a prominent dark line in the spectrum of the brighter
stars of the constellation of Orion. A recent investigator of the
distribution of luminosity in the great nebula of Orion finds that
radiations from nebulium are confined chiefly to the Huygenian region of
the nebula and its immediate neighborhood.
Photography has revealed another extraordinary nebula or group of nebulae
surrounding the stars in the Pleiades, which the deft manipulation of
Barnard has brought to light. All the stars and the nebula are so
interrelated that they are obviously bound together physically, as the
common proper motion of the stars also appears to show. Also in the
constellation Cygnus, Barnard has discovered very extensive nebulosities
of a delicate filmy cloudlike nature which are wholly invisible with
telescopes, but very obvious on highly sensitive plates with long
exposures.
Another class of these objects are the annular and elliptic nebulae which
are not very abundant. The southern constellation Grus, the crane,
contains a fine one, but by far the best example is in the constellation
Lyra. It is a nearly perfect ring, elliptic in figure, exceedingly faint
in small telescopes; but large instruments reveal many stars within the
annulus, one near the center which, although very faint to the eye, is
always an easy object on the photographic plate, because it is rich in
blue and violet rays. The parallax of the ring nebula in Lyra comes out
only one-sixth of that of the planetary nebulae, and the least greatest
diameters of this huge continuous ring are 250 and 330 times the orbit
of Neptune.
Planetary nebulae and nebulous stars are yet another class of nebulae, for
the most part faint and small, resembling in some measure a planetary
disk or a star with nebulous outline. Practically all are gaseous in
composition, and have large radial velocities. Probably they are located
within our own stellar system. The parallaxes of several of them have
been measured by Van Maanen: one of the very small angle 0".023, which
enables us to calculate the diameter of this faint but interesting
object as equal to nineteen times the orbit of Neptune.